Explanation:

The first ionization energy is the energy required to remove one electron from an atom in its gaseous state. It is a measure of the strength of attraction between the nucleus and the valence electrons. Generally, elements with a higher nuclear charge and smaller atomic radius have higher ionization energies.

Comparing the elements:

a) Rb (Rubidium)
- Rubidium is a Group 1 alkali metal with the atomic number 37.
- It has a larger atomic radius and lower nuclear charge compared to the other elements in the list.
- Alkali metals have relatively low ionization energies due to their large atomic size and low effective nuclear charge.

b) Na (Sodium)
- Sodium is also a Group 1 alkali metal with the atomic number 11.
- It has a smaller atomic radius and higher nuclear charge compared to Rubidium.
- Sodium has a higher ionization energy than Rubidium but lower than the remaining elements.

c) K (Potassium)
- Potassium is another Group 1 alkali metal with the atomic number 19.
- It has a larger atomic radius and lower nuclear charge compared to Sodium.
- Potassium has a lower ionization energy than Sodium but higher than Rubidium.

d) Sc (Scandium)
- Scandium is a Group 3 transition metal with the atomic number 21.
- It has a smaller atomic radius and higher nuclear charge compared to the alkali metals.
- Transition metals generally have higher ionization energies due to their smaller atomic size and higher effective nuclear charge.

Conclusion:
Among the given elements, Scandium (Sc) has the highest first ionization energy. This is because it is a transition metal with a smaller atomic radius and higher nuclear charge compared to the alkali metals. The larger attraction between the nucleus and the valence electrons in Scandium requires more energy to remove an electron, resulting in a higher first ionization energy.